The present invention relates to a digital signal transmission system including a modulator and a demodulator, which is based on quadrature amplitude modulation (QAM) and, more particularly, to a digital signal transmission system feasible for large capacity data transmission which uses a great number of levels.
In a carrier wave digital transmission systems art, an attempt has recently been made to increase the frequency utilization efficiency for the purpose of implementing high efficiency data transmission. In a QAM signal transmission system, for example, a current trend is toward the use of greater number of levels and, indeed, the use of 256 levels is under investigation. As regards a digital signal transmission system, when a modulated signal received by a demodulating device is synchronously detected for demodulation, which modulation phase of the modulated signal is represented by a demodulated digital code is not determined unconditionally, as well known in the art. Due to such uncertainty of tuning phase, the combination of P and Q channels changes between a modulating and a demodulating device. Since the linear distortion in a transmission system usually affects each of the P and Q channels to a different degree, the uncertainty of tuning phase causes a demodulated wave-form to change from an optimum wave-form to other various kinds of wave-forms. Consequently, the bit error rate characteristic of a demodulated digital code does not remain constant and fluctuates over a considerable range, affecting the operation and maintenance of digital radio communication channels.
It has been customary to minimize the influence of faulty demodulation stated above by, for example, adjusting the circuitry of a baseband transmission system or by using an equalizer, thereby rendering the bit error rate characteristic constant. However, this kind of implementation is effective only when the number of levels used is as small as sixteen or so. In the case that the number of levels is increased to sixty-four or more with the intension of increasing transmission capacity, the influence of linear distortion in the transmission system becomes so subtle that even the circuitry adjustment of the baseband transmission system, use of an equalizer or like implementation cannot eliminate the faulty modulation problem.
The prior art as described above is disclosed in, for example, "THE 8-LEVEL PSK MODEM WITH COSINE ROLL-OFF SPECTRUM FOR DIGITAL MICROWAVE COMMUNICATIONS" by Y. Tan et al, ICC 76, Vol. II, pages 29-13 to 29-18, "MULTILEVEL PSK MODEMS FOR TDMA SYSTEM" by T. Noguchi et al, EASCON '75 RECORD, pages 139-A to 139-F, and U.S. Pat. No. 4,549,142 (Yoshida).